Engine weight is an important factor when considering the overall cost and performance of a gas turbine engine. For many years attempts have been made to decrease the overall weight of the engine while maintaining or improving engine performance. One manner in which researchers have attempted to reduce the overall weight of the engine is by utilizing composite airfoils in place of the metal airfoils currently employed in most gas turbine engines. Composite airfoils offer a significant weight savings over metal airfoils, however, composite materials have inherently poor resistance to foreign object damage (FOD).
Many types of foreign objects may be entrained in the inlet airflow of an aircraft gas turbine engine ranging from large birds, such as sea gulls, to hailstones, rain, sand, and dust. Damage from foreign objects generally takes two forms. Smaller objects can erode the blade material, causing the aerodynamic shape to change, and degrade the performance of the compressor. Impact by larger leading edge objects can dent or deform the blades. Portions of an impacted blade can also be torn loose and cause secondary damage to downstream blades and other engine components.
The consequences of foreign object damage are greatest in the fan and low pressure compressor sections of turbine engines. However, these components offer the greatest potential in weight reduction due to their large tip diameters, as great as eight feet, and spans in the order of two or more feet.
The vulnerability of composite blades to foreign object damage is due to two factors. First the lightweight matrix materials employed, generally polymeric resins or metals such as aluminum, are relatively soft and do not have high tensile strengths. Second, the high-strength filaments employed in such composites are relatively hard and brittle. As a result, the matrix material is subject to erosion and the fibers are subject to breakage upon foreign object impact.
From this it would appear that some sort of protection system should be provided for these composite blades and vanes. Many such protection systems have been proposed. They include claddings of various compositions applied to the leading edge portion of the entire surface of the blade. One proposed cladding system involves fixing a solid metal sheath over the leading edge of the blade. This procedure, however, requires expensive forming operations and the sheath must ultimately be adhesively bonded to the airfoil as a secondary operation after airfoil manufacture. This process proves to be both costly and time consuming. In addition, solid metal sheaths require stringent surface preparation and priming prior to adhesive bonding, and are subject to environmental degradation of the adhesive bond when in operation. This naturally reduces the life of the protected composite airfoil.
Another proposed method for protecting the leading edge of composite blades and vanes is disclosed and claimed in U.S. Pat. No. 3,892,612, Method for Fabricating Foreign Object Damage Protection for Rotor Blades, Carlson et al. The disclosed and claimed method of U.S. Pat. No. 3,892,612 is directed to a complicated method of applying a protective metal coating to a non-conductive substrate which comprises the steps of (i) incorporating a woven wire mesh into the substrate, by means of a bonding agent which fills the interstices of the mesh, and then abrading the outer surface of the mesh layer to remove the adhesive from its nubs; (ii) applying a thin conductive layer to the bonding agent in the interstices of the mesh with the mesh nubs free of the thin conductive layer; and (iii) electrolytically depositing a metal coating on the wire mesh/conductive layer surface to obtain an essentially uniform thickness coating forming a metallic strip. The above steps are both complicated and time consuming. In addition, it is noted in a later U.S. Pat. No. 4,006,999 entitled, Leading Edge Protection for Composite Blades, Brantley et al., that a metallic strip leading edge protection created by the aforementioned method has demonstrated problems with delamination when impacted by medium-sized birds. This problem, according to the assignee, can result in secondary engine damage as the leading edge protection strip is ingested through the engine and, in addition, engine imbalance at high speeds can cause further damage.